The present invention relates generally to testing integrated circuits, and in particular to testing output resistance of an integrated circuit.
Manufacturers routinely test integrated circuits (IC) such as microprocessors, application specific integrated chips, and memory devices to ensure their quality and specifications.
Testing an output resistance of an IC ensures that the IC communicates properly with other circuits by matching its specified output resistance with the resistance of the others. Output resistance is also commonly referred to as output impedance.
Some conventional ICs have a fixed output resistance. Many conventional methods are available to test the fixed output resistance. Typically, a tester applies a series of test sequences to the ICs to read the fixed output resistance.
Some other conventional ICs have a range of output resistances. Some of these ICs usually have a special pin for connecting to ground through a resistor. Depending on a specific application of the IC, a user can choose different values for the resistor to get the desirable output resistance within the specified range of output resistances.
Because some ICs have a range of output resistances, testing the range of output resistances of these ICs using the method of testing the fixed output resistance of other ICs may not be appropriate.
For these and other reasons stated below, and which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need for a method of testing a range of output resistances of an integrated circuit.
The present invention provides a test system that tests a range of output resistances of an IC.
In one aspect, the test system presents an IC and a resistance circuit. The resistance circuit connects between the IC and a tester. The resistance circuit includes a fixed resistor acted as a match load. To test the output resistance range, a programmable voltage is applied at the tester. The programmable voltage changes the amount of current flowing though the fixed resistor to cause the IC into believing that it is connected to a different resistor. Therefore, the entire range of output resistances can be tested without changing the resistance value of the match load.
In another aspect, a method of testing an IC selects a resistance value, and calculates a current value based on the resistance value. The method calculates a dropped voltage across a resistive network connected between a pin of the IC and ground based on the current value. The method determines a test voltage and applies the test voltage to a tester connected to the pin. Further, the method measures an output resistance of the IC.